1. Field of Invention
The present invention relates to a device and method for a vascular valve for the vascular system and more specifically for the venous vascular system of the human body and a method for forming the venous valve and more specifically for forming the venous valve from a native vein that may have been enlarged diametrically within the patient causing the naturally occurring venous valve to become incompetant.
2. Description of Prior Art
Venous valves are found within native venous vessels and are used to assist in returning blood back to the heart in an antegrade direction from all parts of the body. The venous system of the leg for example includes the deep venous system and the superficial venous system, both of which are provided with venous valves which are intended to direct blood toward the heart and prevent backflow or retrograde flow which can lead to blood pooling or stasis in the leg. Incompetent valves can also lead to reflux of blood from the deep venous system to the superficial venous system and the formation of vericose veins. Superficial veins which include the greater and lesser saphenous veins have perforating branches in the femoral and popliteal regions of the leg that direct blood flow toward the deep venous system and generally have a venous valve located near the junction with the deep system. Deep veins of the leg include the anterior and posterior tibial veins, popliteal veins, and femoral veins. Deep veins are surrounded in part by musculature tissue that assist in generating flow due to muscle contraction during normal walking or exercising. Veins in the lower leg have a static pressure while standing of approximately 80-90 mm Hg and this pressure can be reduced during exercise to 60-70 mm Hg. Despite exposure to such pressures, the valves of the leg are very flexible and can close with a pressure drop of less than one mm Hg. Due to the endothelial covering on the venous valves, they are able to remain patent and resist thrombosis with blood flow rates of less than 50 ml/min found typically in some of the smaller veins of the lower leg. Although the present invention has direct application to the treatment of venous valvular dysfunction of the leg, it is understood that the invention is not limited to this application and can be applied equally well to the treatment of veins throughout the human body as well as other tubular elements of the body requiring a valve.
Veins typically in the leg can become distended from prolonged exposure to excessive pressure and due to weaknesses found in the vessel wall causing the natural venous valves to become incompetent leading to retrograde blood flow in the veins. Such veins no longer function to help pump or direct the blood back to the heart during normal walking or use of the leg muscles. As a result, blood tends to pool in the lower leg and can lead to leg swelling and the formation of deep venous thrombosis and phlebitis. The formation of thrombus in the veins can further impair venous valvular function by causing valvular adherence to the venous wall with possible irreversible loss of venous function. Continued exposure of the venous system to blood pooling and swelling of the surrounding tissue can lead to post phlebitic syndrome with a propensity for open sores, infection, leading to possible limb amputation.
Repair and replacement of venous valves presents a formidable problem due to the low blood flow rate found in native veins, the very thin wall structure of the venous wall and the venous valve, and the ease and frequency of which venous blood flow can be impeded or totally blocked for a period of time. Surgical reconstruction techniques used to address venous valve incompetence involve venous valve bypass using a segment of vein with a competent valve, venous transposition to bypass venous blood flow through a neighboring competent valve, and valvuloplasty to repair the valve cusps. These surgical approaches are described in medical journals and in standard surgical text books. These surgical techniques are highly technique dependent and difficult to perforn by a highly trained vascular surgeon.
The presence of a low or intermittent blood flow rates found in the veins of the lower leg requires that any suitable venous valve replacement contain an endothelial covering to protect the vessel against thrombosis. Blood stoppage for a period of time in contact with most foreign material can result in thrombus formation, and ensuing failure of any venous valve constructed of a polymeric material or some biomaterials.
Quijano describes in U.S. Pat. No. 5,500,014 the use of a biological valvular prosthesis that is obtained from the jugular vein of an animal. The valve is chemically fixed to give it strength. The fixing process tends to cause such valves to be stiff and calcification has been known to occur at flexation sites. Another problem with valves of this type is their lack of forming a stable endothelium with the resulting formation of thrombus when blood flow is impeded or temporarily blocked. Others have tried constructing venous valves out of a biological tissue material obtained from another species or the same species. Some tissues that have been used include pericardium and venous tissue treated with a crosslinking treatment. These devices have suffered problems associated with calcification, tissue degradation, tissue rejection, acute thrombosis, long term thrombosis, and valvular failure due to mechanical dysfunction.
Another vascular prosthesis that is constructed out of polymeric or metallic components is described by Camilli is U.S. Pat. No. 5,358,518. He describes a movable rigid or semi-rigid plate that pivots and allows unidirectional blood flow through the venous tube. The biomaterials used in the device of Camilli will not form a stable endothelium on the blood flow surface; due to the low blood flow and often times interrupted blood flow found within the venous system, this device will be prone to thrombosis and failure.
Laufer (U.S. Pat. No. 5,810,847) describes an appliance that is constructed out of a biomaterial that is attached to the cusps of an existing venous valve. Such a polymeric appliance will have a propensity to thrombosis in the low blood flow conditions found in the venous system. Also, many patients with venous problems do not have suitable valves onto which the appliance described by Laufer can be attached.
Lane describes in U.S. Pat. Nos. 5,147,389 and 4,904,254 and Shifrin describes in U.S. Pat. No. 5,476,471 devices that are intended to surround the outer surface of incompetent or insufficient venous valves. These devices are intended to reduce the diameter of the vessel in the region of the incompetent valve allowing the natural cusps to approximate each other leaving the valve commissures intact. For these devices to work properly, the valves of the vein must not be adherent to the vessel wall, this adherent condition is often found when the vessel is exposed to deep venous thrombosis. This significantly limits the patient population that can benefit from a device of this type.
A treatment for venous valvular dysfunction in patients that have had their vein wall distended and their valves irreversibly damaged is needed. The treatment should involve autologous tissue such that an endothelial layer is present in blood contact and thrombosis due to low flow is not of concern. The treatment should be easily performed so that patients with tissue edema and ulcers can tolerate the intervention and heal the area being accessed. The treatment should be applicable to those patients who have had deep venous thrombosis, phlebitis, or other vascular trauma and have irreversibly lost venous valvular function.
The present device and method for forming a venous valve overcomes the disadvantages of prior art prosthetic venous valves constructed out of crosslinked biological tissue, polymeric, or other biomaterials and it also overcomes problems associated with the surgical repair of venous valves. The venous valve of the present invention is constructed directly from the distended vein of a patient and it does not require the presence of an existing venous valve. Since the valve of the present invention is being constructed from autologous vein tissue with the natural endothelium of the vein in immediate contact with the blood, thrombosis such as that associated with biologic, polymeric, or other biomaterial constructed prosthesis is not a major concern. The venous valve of the present invention is configured from a segment of distended vein forming a new venous valve within the distended vein. Problems associated with irreversible valvular destruction or adhesion to the vessel wall due to deep venous thrombosis, phlebitis, or other vascular trauma will not influence the formation of the valve of the present invention within a distended vein. The method of forming the venous valve of the present invention is intended to be much simpler than venous valve bypass, valvular transposition, or valvuloplasty as performed by the peripheral vascular surgeon. The present method of forming the venous valve is intended to require only a minimally invasive surgery involving an easier and direct surgical procedure that can be applied to almost any distended vein.
The venous valve of the present invention involves identifying a segment of cylindrical tubular shaped distended vein and forming it into a venous conduit with a working valve in it. The venous valve has an overlap region that contains a through-flow member to allow antegrade blood flow through the venous valve and a sinus member which provides a cavity that is filled during the initiation of retrograde flow. A single valve cusp is located between the overlap through-flow member and the overlap sinus member. The valve cusp is capable of moving toward the sinus member to allow antegrade flow and moving to block the through-flow portion during the initiation of retrograde or back flow. The venous valve can also have an inlet transition region and it can have an outlet transition region. The transition regions are intended to provide a gradual tapering of the blood flow path from the distended vein to the smaller diameter of the overlap region. The blood flow surface of the through-flow and sinus portions of the overlap region, the transition regions, and the valve cusp are all endothelialized with the natural endothelium found in the native distended vein.
The structure of the venous valve is most easily and clearly described using cylindrical coordinates to describe the native distended vein and its relationship to various aspects of the venous valve that is formed from it. A cross section taken through the distended vein on the inlet side of the venous valve and facing downstream can be assigned a zero degree radian that intersects with the vein wall at a zero degree wall. Similarly one can identify a 90, 180, 270, or another degree wall along the vein wall at the inlet end of the overlap region. A zero degree line can be identified as that portion of the distended vein wall that extends axially and passes through the zero degree wall. The zero degree line describes venous wall material that may be sewn, sutured, stapled, bonded by adhesive, or attached in another way to another portion of venous wall material. The zero degree line can include the inner or outer surface of the vessel wall or it can include the entire vessel wall. Similarly, 90, 180, and 270 degree lines can also be identified. A first quadrant can be identified as the venous vessel wall that extends from the zero degree line to the 90 degree line. Similarly, a second, third, and fourth quadrant can also be identified.
For ease of describing the present invention, the vessel wall has been divided into quadrants, however it is understood that the vessel could be divided into a much greater number of sectors, each sector describing a smaller angle than 90 degrees and a smaller wall surface area than is occupied by a quadrant. Also, for ease of understanding, a wall line has been defined for the purposes of the drawings as extending approximately parallel to the centerline or axis of a tubular vessel. It is understood that for the venous valve of the present invention that a wall line is only required to have a component in the axial direction.
The venous valve of the present invention is intended to direct blood flow in a single antegrade direction from upstream to downstream, from its inlet to its outlet, toward the heart. The overlap region of this invention has an inlet and outlet end and an axial overlap length. In one embodiment of the present invention a segment of distended vein with an axial overlap length is identified. The zero degree line of the vein is brought into contact with the 180 degree line and the vein walls are attached using an attachment means which includes suturing or sewing, staples, adhesives or other bonding agents, thermal or other forms of welding, or other physical or chemical attachment mechanisms. It is understood that reference to the zero degree line, 180 degree line or other degree lines, points, or quadrants are approximate references and deviations from these lines, points, or quadrants are allowed within the framework of the present invention. For ease of understanding the invention is being described such that a wall line extending approximately parallel to the axis and extending throughout the overlap region is attached to another parallel wall line. It is understood that the wall lines need not be parallel to the axis and they need not extend throughout the overlap region to describe the present invention. To describe the present invention only a portion of such a wall line need be attached to a portion of another wall line. The 90 and 270 degree lines are brought into contact and the walls are attached using attachment means. At the inlet end of the overlap region, the first, second, and fourth quadrants are attached using attachment means. At the outlet end of the overlap region, the first and fourth quadrants are attached together.
The overlap region has a through-flow member defined by the third and fourth quadrant. At the outlet end of the overlap region is an opening between the first and second quadrant that provides entry into the sinus member of the overlap region. The first and fourth quadrant which are attached at the outlet end of the overlap region serve as the valve cusp. This attachment is not required to occur at the outlet end of the overlap region. It is understood that a portion of the first and fourth quadrants are attached together to form a member of the valve cusp. The cusp moves into approximation with the second quadrant during antegrade flow, and moves into approximation with the third quadrant during the initiation of retrograde or back flow allowing the sinus cavity or member to fill with blood. During antegrade blood flow through the venous valve, blood is directed through the through-flow member from the inlet to the outlet of the overlap region. This occurs for example in the legs during muscular contraction associated with walking or running or when the leg is in a reclined position and the pulsing of the heart moves the blood throughout the body including blood return from the legs. During the period of time that blood is not moving in an antegrade direction such as during standing or between periods of muscle contraction, blood is prevented from moving in a retrograde direction or from backflow or reflux by the venous valve. At the outlet end of the overlap region some backflow will go into the sinus member of the venous valve causing the sinus member to fill. This occurs as retrograde blood flow through the through-flow member generates fluid shear stresses upon the valve cusp causing it to move in the direction of flow. Due to geometrical constraints of the valve cusp with the inner and outer walls of the overlap region, the valve cusp also moves away from the second quadrant and allows blood to enter the sinus member of the overlap region. A small pressure difference between the blood at the inlet end of the sinus member and the outlet end of the sinus member which is dead ended and will not allow further retrograde blood flow, provides the driving force for blood entry into the sinus member. The valve cusp will move toward the third quadrant causing the through-flow member to close and stop further backflow or retrograde flow.
The four quadrants of the overlap region can be supported either within the walls or on the surface of the walls to resist possible stretching. A suture, thread, ribbon or other supportive means can be sewn, sutured, bonded, or placed along the third quadrant of the through-flow member at the inlet end to ensure that the entrance to the overlap region does not enlarge with continued exposure to pressure. Similarly, a supportive means can be attached to or placed along the second quadrant of the sinus member or the third quadrant of the through-flow member at the outlet end to ensure that further venous distension does not prevent the valve cusp from providing closure to the through-flow member. Additionally, further supportive means can be placed throughout any of the four quadrants to support both the through-flow member and sinus member.
The venous valve of the present invention can have an inlet transition region to connect the inlet end of the overlap region to the cylindrically shaped distended vein and an outlet transition region to connect the outlet end of the overlap region to the cylindrically shaped distended vein. These transition regions provide a tapered shape for blood to follow from the large distended vein to the smaller diameter for the through-flow member of the overlap region and back to the large diameter of the distended vein. Several possible ways of forming the transition regions are possible and are provided in the present invention in various embodiments to the invention.
The inlet transition region will be described at a location adjacent to the inlet end of the overlap region and at a location approximately half way between the inlet end of the overlap region and the cylindrically shaped native distended vein. Near the inlet end of the overlap region, the transition region has its zero degree wall in approximation to the 180 degree wall, and the 90 degree wall is in contact with and attached to the 270 degree wall. At the location halfway between the overlap region and the cylindrically shaped vein, approximately the 45 degree wall is attached to the 315 degree wall. This provides approximately a linear tapering of the diameter of the inlet transition region from the overlap region to the cylindrically shaped vein. It is understood that attachments can be made continuously along the transition region, not just at the two points that have been identified. Any excess material from the first and fourth quadrant that extends into the lumen of the transition region or extends out from the lumen of the transition region can be trimmed off or removed, provided that a leak tight attachment has been made along the transition region. The outlet transition region can be constructed in a manner similar to that of the inlet transition region.
An alternate method of constructing the inlet transition region again can be examined at two locations with interpolation or extrapolation of the results to other locations along the transition region. At the junction of the inlet transition region with the inlet end of the overlap region the zero degree point is in contact with and attached to the 180 degree point and the first quadrant can be attached to the second quadrant. At a location approximately half way between the inlet end of the overlap region and the cylindrically shaped distended vein, the 45 degree point is in contact with and attached to the 135 degree point. It is anticipated that a continuum of points are attached along the transition region such that a generally tapered shape for the transition region is formed from the overlap region to the cylindrically shaped vein.
In another embodiment for forming the overlap region, a segment of cylindrically shaped distended vein is placed into a flattened conformation and the first and fourth quadrants are attached together along the inlet end of the overlap region and cut at the inlet end of the overlap region through the first and fourth quadrant. Similarly, the distended vein is attached together along the outlet end of the overlap region and cut through the first and fourth quadrants of the outlet end of the overlap region. The zero degree line is placed into contact with and attached to the 180 degree line from the inlet end to the outlet end of the overlap region. This is accomplished by inverting the first and fourth quadrants into the second and third quadrants. This forms two separate tubes in the overlap region, one for through flow of blood between the third and fourth quadrant and one to serve as the sinus member of the overlap region between the first and second quadrant. At the inlet end of the overlap region the first and second quadrants are attached together. This is necessary to prevent antegrade blood flow from entering into the sinus member of the overlap region. The 90 and 270 degree lines are attached together from the inlet end to the outlet end of the overlap region to hold the sinus member into immediate contact with the through-flow member of the overlap region. This provides a single lumen between the third and fourth quadrant for through flow through this member of the overlap region. At the outlet end of the overlap region is located the leading edge of the single valve cusp that is made up of the first and fourth quadrants of the overlap region. The valve cusp is completely endothelialized along with all blood contact surfaces of the venous valve. The inverted nature of the leading edge provides this embodiment with a particularly good resistance to thrombosis. The walls of the valve can be supported as described earlier with the other embodiments.
The inlet transition regions for the embodiment just discussed can be formed by attaching the first and fourth quadrants together on a line that extends from a point of attachment of the 90 and 270 degree walls at the inlet end of the overlap region along a beveled pathway to a zero degree wall on the cylindrically shaped distended vein. The beveled or tapered pathway provides a smooth blood flow transition from the distended vein to the overlap region. Any excess venous tissue that extends from the beveled attachment line to the zero degree line in the transition region can be trimmed off or removed. The outlet transition region can be similarly constructed.
It is noted that the construction of the venous valve of this invention requires some means for attachment between various quadrants of the venous wall. In forming the attachment of the zero degree line with the 180 degree line of the first embodiment, or attaching one quadrant to another along an inlet or outlet end of the overlap region, or for forming many other of the wall attachments or wall cuts, the distended vessel can be placed in a flattened position to allow the attachments to be made easier. The venous valve of this invention lends itself well to a mechanism that will assist the surgeon in making the attachments and cuts and allow the formation of this venous valve to be made quickly and consistently.
It is understood that the inlet and outlet transitions region embodiments can be interchanged with each other when possible. It is understood that other possible variations of the overlap region or transition regions may exist that are not shown by the drawings but are indeed taught by this patent application and should be considered as part of the teachings of this disclosure. The teachings of this patent application are not limited to the drawings and embodiments included herein.
It is further understood that the venous valve of the present invention can be constructed out of biological tissue such as venous tissue from animal sources, autologous tissue such as pericardium or venous tissue from another part of the body, or polymeric materials such as those used in vascular grafts. A venous valve of the present invention can be constructed as per the methods taught in the present disclosure and the resultant venous valve can be implanted interpositionally into a vein of a person at the venous site that requires a functioning venous valve. An embodiment such as this provides the advantage of simplicity of design and ease of formation.